Organic field-effect transistors (OFETs) based on organic semiconductor (OSC) thin films show promise as building blocks for low-cost, large-area and flexible electronics for applications such as displays, smart cards, radio-frequency identification tags, and sensors. Most of the OFETs fabricated to date exhibit unipolar conduction with a single carrier type (holes or electrons, p or n respectively). This makes complementary circuit elements, which are more power-efficient and lower-noise than unipolar analogues, challenging to implement because of the need to pattern more than one OSC material. Recent attempts to realize single-component OFET complementary inverters have focused on OSC-metal electrode interface modifications to control the carrier injection. They have been operated in high vacuum or under inert atmosphere either with relatively unstable low work-function metals such as calcium employed as the source and drain electrodes and calcium as chemical dopants of the OSC, or molecules designed to optimize the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels.
While an inversion layer (mobile charges of the minority carrier type) is often obtained in silicon semiconductor devices using a metal-insulator-semiconductor or metal-oxide-semiconductor structure where the metallic gate is biased to supply the required electric field, organic semiconductor thin films generally do not show inversion operation (intended here as the accumulation of the less probable carrier type based on orbital energy levels) because of the large gate voltage required to bend the bands and fill traps. In most OFETs, numerous traps will occur near the gate dielectric interface or around the OSC grain boundaries. The limitation of charge transport by only one carrier type is generally ascribed to this effective trapping of the other carrier type. The trapping limitation could be lessened by using ultrapure, high-quality materials in single crystal devices and the realization of the intrinsic (not limited by disorder) charge transport on the OSC single-crystal surface.